1. Field of the Invention
The present invention relates to a novel mold for producing foamed articles. Specifically, an aspect of the present invention relates to an improved mold including a vent capable of providing necessary venting of the mold while minimizing extrusion of raw material into the vent, thus reducing wastage. In its most preferred embodiment, the present invention also eliminates the requirement to remove the extruded raw material from the molded article prior to application of a finish (e.g. trim) cover.
The present invention also relates to a novel method of molding articles which reduces wastage resulting from raw material being extruded into the vent during venting of the mold. In its most preferred embodiment, the method reduces or eliminates the requirement to remove the extruded material prior to application of a finish cover.
2. Description of the Prior Art
Many articles are manufactured by placing a raw material into a cavity in a mold wherein the raw material undergoes a physical change (e.g. it expands or foams) and the article produced thus acquires the shape of the cavity. In particular, this technique is commonly employed for producing foamed articles made from polymeric foams such as polyurethane foam, latex (e.g. natural and styrene-butadiene rubber) foam and the like.
For example, automotive seats are commonly manufactured from polyurethane cushions which are molded to shape and then covered with a vinyl, cloth or leather finish cover. Polyurethane foams are somewhat unique in that foaming and at least a portion of the polymerization process occur simultaneously. Thus, in the production of polyurethane foam using, for example, a conventional cold foam technique, a typical formulation comprises:
1. Polyol
2. Water
3. Tetramethyl ethane diamine
4. Dimethyl ethanol amine
5. Polyisocyanate
The mixture is dispensed into a mold using a suitable mixing head, after which the mold is then closed to permit the expanding mass within it to be molded. Accordingly, it is convenient generally to refer to the mixture initially dispensed into the mold as "a liquid foamable polymeric composition" or, in this case, "a liquid foamable polyurethane composition". As the composition expands in the mold, polymerization occurs and the polymer so formed becomes solidified.
When molding a liquid foamable polymeric composition to form articles such as polyurethane foam articles, it is conventional to use a clam-shell mold comprising a bottom mold and a top mold which, when closed, define a mold cavity. The mold is opened, the liquid foamable polyurethane composition is dispensed into the mold cavity and the mold is closed as a chemical reaction causes the composition to expand. After the mold is closed, the composition expands to fill the interior cavity of the mold. Alternatively, the composition may be dispersed into a closed mold. In either case, as the polymerization reaction is completed, the foam cures and permanently assumes the shape of the mold cavity.
As is known to those of skill in the art, it is essential during this process that the mold be adequately vented to allow the air present in the mold to exit the mold as the foamable composition expands. Further, it is essential to allow a portion of the gases (typically CO.sub.2 in the production of polyurethane) generated during polymerization to exit the mold.
Failure to adequately vent the mold results in defective molded articles exhibiting symptoms of improper foaming such as surface hardening (or foam densification) and/or void formation in the finished article due to trapped gas or air bubbles. At the other extreme, excess venting of the mold will also result in defective molded articles due to collapse of the foam prior to curing; this phenomenon is often referred to as the `souffle` effect. Thus, proper venting of molds is an important factor in producing acceptable molded articles.
Typically clam-shell molds are designed with drilled or cut passages in the top mold to provide vents. Locating, sizing and deciding upon the number of these vents is a matter of some skill on the part of mold designer and the production engineers, and is often an iterative procedure with more vents being added to various locations or other vents being blocked-off after test runs have been made.
During molding operations some liquid foamable polymeric composition which moves into the vent is wasted. It is generally desired to minimize the amount of wasted material (also known as "flash", "mushrooms", "buds", "pancakes" and the like) for two masons, namely (1) the wasted material adds to the overall expense of producing the finished article, and (2) the wasted material must be removed from the molded article prior to the finish cover being applied, thereby necessitating additional labour and the costs associated therewith.
Accordingly, mold designers and production engineers are continually striving to optimize the compromise between providing enough venting at the proper locations while avoiding excess venting and minimizing material wastage during venting.